It is a pre-set striker fired semi-automatic pistol, meaning the trigger system is of the hammerless short double-action-only type.
A full size 9mm pre-loaded striker pistol, known as the FN 509, was introduced in early 2016 with it being part of the XM17 Modular Handgun System competition.
In 2015, officer Richard Vankeuren, a 20-year veteran of the Arizona Department of Public Safety, bumped his holstered FN FNS-40 into a bag he was carrying and the weapon went off, shooting him in the leg.
The Baltimore police also claimed that they had encountered three separate incidents of the roll pin in the frame breaking, causing the trigger to completely fall out of the firearm.
On October 30, 2018, Baltimore County Police Chief Terrence Sheridan issued an Emergency Justification for a Replacement Service Weapon.
FN immediately addressed this by replacing the striker with a newer, improved design in each FNS-40 for both the Arizona DPS and the BCPD, at no cost to either agency.
FN also posted a Service Bulletin to the public with an offer of a free striker upgrade for those who wished to send their pistol in.
FN also claimed that the trigger pin is a part that requires periodic maintenance, and that in all three cases, the firearm was quickly reassembled.